The present invention relates generally to injection molding and, more particularly, it relates to the production of casual footwear of the slipper type including an upper of a knitted fabric, preferably of the stretch type, a woven insole, and an integral, injection-molded sole of a thermoplastic material.
Footwear having an injection-molded bottom or sole portion directly attached to a fabric upper has been produced for many years. Generally, an injection-molding machine is provided with a fixed, partial mold assembly in the fixed platen, including a depression in the shape of a shoe sole and of a depth as great as the desired sole thickness. This partial mold is connected through a sprue to a plastic injector nozzle, and is also suitably gated.
A last assembly is spaced vertically above the partial mold and, upon actuation of the machine, moves downwardly thereover. The last is generally foot-shaped and, of course, is sized to fit closely over the mold cavity, capping it, and leave a closed space of the desired sole thickness and configuration. Provision is made for maintaining the liquid thermoplastic at desired temperatures as it is injected, flows into the mold and solidifies, this being accomplished by heating means in the melt and chillers in the mold.
In practice, an operator fits an upper over the last (or, more typically, a right-and-left pair of lasts) and actuates the machine. The last drops over the mold, the plastic is injected and, after a predetermined cooling period, the last moves back to its upper position and the completed article is removed therefrom. The time period from one machine activation to the next is referred to as the mold cycle.
The duration of the mold cycle depends to a large extent on the required cooling time for the thermoplastic, the speed of plastic injection, and, to a lesser extent, on the complexity of the manual loading operation (as noted hereinbelow, much of this footwear includes, in addition to an upper and a sole, preformed insoles, peripheral molded bands, cleats, etc.). Of course, if the operator is required to tend a plurality of machines and is not present at the end of each operation, mold cycle time is further increased.
Thus, with a minimum cooling time of about 1.5 minutes, which implies a minimum mold cycle of about 2.5 minutes, it will be appreciated that such operations are severly limited in terms of labor productivity and capital productivity, even though one can trade off one at the expense of the other.